1. Field of the Invention
This invention relates generally to the construction of concrete slabs. More particularly, the invention relates to an improved joint edge assembly that protects the joint edges and allows the joint to self-open as the concrete shrinks during hardening.
2. Related Art
For logistical and technical reasons, concrete floor slabs are made up of a series of individual blocks. The interface where one block meets another is termed a joint. Freshly placed concrete shrinks considerably as it hardens as the chemical reaction between the cement and the water occurs, i.e., hydration. As the concrete shrinks, tensile stress accumulates in the concrete, therefore, the joints should be free to open and thus allow shrinkage to occur without damaging the slab.
The joint openings, however, create discontinuities in the slab surface, which can cause the wheels of forklift trucks and other vehicles to impact the joint edges and chip small pieces of concrete from the edge of each slab. This damage to the edges of slabs is commonly referred to as xe2x80x9cjoint spalling.xe2x80x9d Joint spalling often interrupts the normal working operations of many facilities by slowing down forklift and other truck traffic, and/or causing damage to trucks and the carried products. Severe joint spalling can even cause loaded forklift trucks to be overturned and can be dangerous to employees. Furthermore, joint spalling can be very expensive to repair.
For these reasons, it is advantageous to protect the joint edges against spalling with steel bars or angles. Commonly used details illustrating the use of hot rolled steel bars (or angles) are shown in the American Concrete Industry (ACI) technical manuals 302 and 360. However, the standard installation procedure for these steel bars or angles is both time-consuming and expensive. The conventional procedures typically includes the following steps: (1) a temporary edge form is erected; (2) the first bar (or angle) is attached to the edge form; (3) the first concrete slab is cast; (4) the form is removed; (5) the second bar (or angle) is tack welded to the first; (6) the second concrete slab is cast; and (7) the tack welds are removed by grinding. Importantly, the quality control of the tack welding and the timing of the tack weld grinding are critical to the joint performance. If a weld is not completely removed by grinding, or if grinding is not completed shortly after the second slab is cast, then the joint remains locked together and tensile stress accumulates in the slabs, which often leads to unacceptable slab cracking.
For at least the foregoing reasons, an improved joint edge assembly that protects the joint edges of the concrete slab, and allows the joint to self-open as the concrete shrinks during hardening would be desirable.
The invention is an improved joint edge assembly that protects the joint edges of concrete slabs and allows the joint to self-open as the concrete shrinks during hardening. The apparatus comprises a longitudinal joint rail, made up of two elongated joint edge members. The elongated joint edge members are typically steel bar sections, but can be any similar suitable material. The sections are connected to one another along their length by two sets of connectors. The first set secures the sections during shipping and placement, and are removed before pouring the adjacent concrete slab. The second set of connectors remain throughout the concrete pouring operation and include release elements that self-release the joint edge members from each other under the force of the slabs shrinking during hardening, thus allowing the joint to open. The joint rail is supported above the ground surface by a mounting bracket attached to temporary formwork seated on the ground surface. A plurality of studs extend from the elongated joint edge members into the region where the slab is to be poured such that, upon hardening of the concrete slab, the studs are integrally cast within the body of the slab. One or more dowel aligners may be integrated into the form assembly to allow dowels to be accurately positioned within the adjacent slab sections. Alternatively, a base and sleeve may be used where a load plate is employed between adjacent slabs rather than dowels.
When the first of the adjacent slab sections is poured, the claimed form assembly restrains the wet concrete. Preferably, studs extending from the longitudinal joint rail become embedded in the concrete slab, providing a positive mechanical connection between the slab and the form assembly when the concrete hardens. Before pouring the adjacent slab, the connectors used to secure the longitudinal joint rail during shipping and placement are removed such that the connectors containing the self-release elements remain. This step is best taken after the concrete has hardened sufficiently to support the longitudinal joint rail. Further, at this point the dowels or load plates are placed, if desired, using the aligners that were cast into the first concrete slab. After pouring the adjacent slab, the studs extending from the longitudinal joint rail into the adjacent slab region become embedded in the adjacent concrete slab, providing a positive mechanical connection between the adjacent slab and the form assembly. As the chemical reaction between the cement and the water occurs, i.e., hydration, the concrete hardens and shrinks. As the slabs shrink away from one another, the self-release elements allow the elongated joint edge members to separate from one another. If desired, the gap formed by the separated joint edge members may be filled with a sealant.